Processes for forming backplanes for electro-optic displays

ABSTRACT

A non-linear element is formed on a flexible substrate by securing the substrate to a rigid carrier, forming the non-linear element, and then separating the flexible substrate from the carrier. The process allows flexible substrates to be processed in a conventional fab intended to process rigid substrates. In a second method, a transistor is formed on a insulating substrate by forming gate electrodes, depositing a dielectric layer, a semiconductor layer and a conductive layer, patterning the conductive layer to form source, drain and pixel electrodes, covering the channel region of the resultant transistor with an etch-resistant material and etching using the etch-resistant material and the conductive layer as a mask, the etching extending substantially through the semiconductor layer between adjacent transistors. The invention also provides a process for forming a diode on a substrate by depositing on the substrate a first conductive layer, and a second patterned conductive layer and a patterned dielectric layer over parts of the first conductive layer, and etching the first conductive layer using the second conductive layer and dielectric layer as an etch mask. Finally, the invention provides a process for driving an impulse-sensitive electro-optic display.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from Application Ser. No. 60/375,246,filed Apr. 24, 2002 and Application Ser. No. 60/376,603, filed Apr. 30,2003. This application is also related to application Ser. No.09/565,413, filed May 5, 2000; application Ser. No. 09/904,109, filedJul. 12, 2001 (Publication No. 2002/0106847, now U.S. Pat. No.6,683,333); copending application Ser. No. 09/904,435, filed Jul. 12,2001 (Publication No. 2002/0060321), and copending application Ser. No.10/065,795, filed Nov. 20, 2002 (Publication No. 2003/0137521). Thisapplication is also related to copending application Ser. No.10/249,618, of even date herewith, entitled “Backplanes for displayapplications, and components for use therein” (Publication No.2003/0222315). The entire contents of the aforementioned applicationsare herein incorporated by reference. The entire contents of all UnitedStates Patents and published Applications mentioned below are alsoherein incorporated by reference.

BACKGROUND OF INVENTION

The present invention relates to processes for forming backplanes forelectro-optic (electronic) displays. This invention also relates tocertain improvements in non-linear devices for use in such backplanesand to processes for forming such non-linear devices.

The term “electro-optic” as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin published U.S. Patent Application No. 2002/0180687 that someparticle-based electrophoretic displays capable of gray scale are stablenot only in their extreme black and white states but also in theirintermediate gray states, and the same is true of some other types ofelectro-optic displays. This type of display is properly called“multi-stable” rather than bistable, although for convenience the term“bistable” may be used herein to cover both bistable and multi-stabledisplays.

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedto applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. No. 6,301,038, International Application Publication No. WO01/27690, and in copending application Ser. No. 10/249,128, filed Mar.18, 2003 (now U.S. Pat. No. 6,950,220. This type of medium is alsotypically bistable.

Another type of electro-optic display, which has been the subject ofintense research and development for a number of years, is theparticle-based electrophoretic display, in which a plurality of chargedparticles move through a suspending fluid under the influence of anelectric field. Electrophoretic displays can have attributes of goodbrightness and contrast, wide viewing angles, state bistability, and lowpower consumption when compared with liquid crystal displays.Nevertheless, problems with the long-term image quality of thesedisplays have prevented their widespread usage. For example, particlesthat make up electrophoretic displays tend to settle, resulting ininadequate service-life for these displays.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspension medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;6,521,489; 6,531,997; 6,535,197; 6,538,801; and 6,545,291; and U.S.Patent Applications Publication Nos. 2002/0019081; 2002/0021270;2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677; 2002/0090980;2002/0106847; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0145792;2002/0154382, 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378;2003/0011560; 2003/0011867; 2003/0011868; 2003/0020844; 2003/0025855;2003/0034949; 2003/0038755; and 2003/0053189; and InternationalApplications Publication Nos. WO 99/67678; WO 00/05704; WO 00/20922; WO00/26761; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/67110; WO00/67327; WO 01/07961; and WO 01/08241.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display in whichthe electrophoretic medium comprises a plurality of discrete droplets ofan electrophoretic fluid and a continuous phase-of a polymeric material,and that the discrete droplets of electrophoretic fluid within such apolymer-dispersed electrophoretic display may be regarded as capsules ormicrocapsules even though no discrete capsule membrane is associatedwith each individual droplet; see for example, the aforementioned2002/0131147. Accordingly, for purposes of the present application, suchpolymer-dispersed electrophoretic media are regarded as sub-species ofencapsulated electrophoretic media.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes; andother similar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the suspending fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, International Applications Publication No. WO 02/01281, andpublished U.S. Application No. 2002-0075556, both assigned to SipixImaging, Inc.

The aforementioned types of electro-optic displays are bistable and aretypically used in a reflective mode, although as described in certain ofthe aforementioned patents and applications, such displays may beoperated in a “shutter mode” in which the electro-optic medium is usedto modulate the transmission of light, so that the display operates in atransmissive mode. Liquid crystals, including polymer-dispersed liquidcrystals, are, of course, also electro-optic media, but are typicallynot bistable and operate in a transmissive mode. The backplanes of thepresent invention may be used with both reflective and transmissivedisplays, including conventional liquid crystal displays.

Whether a display is reflective or transmissive, and whether or not theelectro-optic medium used is bistable, to obtain a high-resolutiondisplay, individual pixels of a display must be addressable withoutinterference from adjacent pixels. One way to achieve this objective isto provide an array of non-linear elements, such as transistors ordiodes, with at least one non-linear element is associated with eachpixel, to produce an “active matrix” display. An addressing or pixelelectrode, which addresses one pixel, is connected to an appropriatevoltage source through the associated non-linear element. Typically,when the non-linear element is a transistor, the pixel electrode isconnected to the drain of the transistor, and this arrangement will beassumed in the following description, although it is essentiallyarbitrary and the pixel electrode could be connected to the source ofthe transistor. Conventionally, in high resolution arrays, the pixelsare arranged in a two-dimensional array of rows and columns, such thatany specific pixel is uniquely defined by the intersection of onespecified row and one specified column. The sources of all thetransistors in each column are connected to a single column electrode,while the gates of all the transistors in each row are connected to asingle row electrode; again the assignment of sources to rows and gatesto columns is conventional but essentially arbitrary, and could bereversed if desired. The row electrodes are connected to a row driver,which essentially ensures that at any given moment only one row isselected, i.e., that there is applied to the selected row electrode avoltage such as to ensure that all the transistors in the selected roware conductive, while there is applied to all other rows a voltage suchas to ensure that all the transistors in these non-selected rows remainnon-conductive. The column electrodes are connected to column drivers,which place upon the various column electrodes voltages selected todrive the pixels in the selected row to their desired optical states.(The aforementioned voltages are relative to a common front electrodewhich is conventionally provided on the opposed side of theelectro-optic medium from the non-linear array and extends across thewhole display.) After a pre-selected interval known as the “line addresstime” the selected row is deselected, the next row is selected, and thevoltages on the column drivers are changed to that the next line of thedisplay is written. This process is repeated so that the entire displayis written in a row-by-row manner. Thus, in a display with N rows, anygiven pixel can only be addressed for a fraction 1/N of the time.

Processes for manufacturing active matrix displays are well established.Thin-film transistors, for example, can be fabricated using variousdeposition and photolithography techniques. A transistor includes a gateelectrode, an insulating dielectric layer, a semiconductor layer andsource and drain electrodes. Application of a voltage to the gateelectrode provides an electric field across the dielectric layer, whichdramatically increases the source-to-drain conductivity of thesemiconductor layer. This change permits electrical conduction betweenthe source and the drain electrodes. Typically, the gate electrode, thesource electrode, and the drain electrode are patterned. In general, thesemiconductor layer is also patterned in order to minimize strayconduction (i.e., cross-talk) between neighboring circuit elements.

Liquid crystal displays commonly employ amorphous silicon (“a-Si”),thin-film transistors (“TFT's”) as switching devices for display pixels.Such TFT's typically have a bottom-gate configuration. Within one pixel,a thin-film capacitor typically holds a charge transferred by theswitching TFT. Electrophoretic displays can use similar TFT's withcapacitors, although the function of the capacitors differs somewhatfrom those in liquid crystal displays; see the aforementioned copendingapplication Ser. No. 09/565,413, and Publications 2002/0106847 and2002/0060321. Thin-film transistors can be fabricated to provide highperformance. Fabrication processes, however, can result in significantcost.

In TFT addressing arrays, pixel electrodes are charged via the TFT'sduring a line address time. During the line address time, a TFT isswitched to a conducting state by changing an applied gate voltage. Forexample, for an n-type TFT, a gate voltage is switched to a “high” stateto switch the TFT into a conducting state.

Many electro-optic materials require application of a drive voltage fora significant switching time (typically of the order of 10⁻² to 10⁻¹seconds) to effect a transition between their two extreme opticalstates. For high resolution displays containing at least (say) 100 rowsand columns, if a reasonable scan rate is to be maintained, the periodfor which an individual pixel is addressed during a single scan is muchless than the switching time of the electro-optic medium, andaccordingly much of the switching of a pixel is effected by the voltagewhich remains on the pixel electrode between successive times ofaddressing the pixel (i.e., while other rows of the display are beingaddressed). This remaining voltage gradually decays due to currentpassing through the electro-optic material of the pixel and any currentleakage through the non-linear element. The rate at which this decayoccurs can be reduced (and the average voltage applied to the pixelduring one complete scan of the display thus increased—this is commonlyreferred to as “increasing the voltage holding capacity” of the pixel)by connecting the pixel electrode to a capacitor.

At least some of the aforementioned electro-optic media can be madesufficiently flexible to permit their use in flexible displays basedupon flexible substrates such as metal or polymeric films. However,manufacturing flexible microelectronic backplanes for such displayspresents many challenges. Flexible substrates such as thepolyimide-over-steel substrates described in the aforementioned2002/0019081 will likely require specialized tooling for substratehandling. This is problematic for two reasons. Firstly, the requiredtooling does not exist. Secondly, a substantial investment by anexisting display manufacturer will be required. This investment includestaking an existing glass-fab off-line, retrofitting the equipment,bringing the fab back on-line, and climbing the yield curve again. Afterretrofitting a fab for flexible substrates, it may not be possible tosimultaneously process glass substrates due to the differences in thefixtures required. Thick steel substrates may not require as muchspecialized tooling for handling; however, display manufacturers maystill be hesitant to process steel substrates due to contaminationconcerns and other issues. In any case, display manufacturers may beunwilling to make the required investment during the early stages offlexible display development.

In one aspect, the present invention allows a flexible display to bemanufactured in an existing glass fab with virtually no changes to thefacility. The fab could stay on-line and would be able to simultaneouslyproduce flexible and glass backplanes. The invention provides a processin which a microelectronic display is fabricated on a rigid carrier andthen released to produce a flexible display.

In another aspect, this invention seeks to reduce one major factor inthe cost of preparing active matrix backplanes for electro-opticdisplays, namely the patterning steps; the present invention seeks toreduce the number of patterning steps needed.

In another aspect, this invention seeks to reduce the cost of diodebackplane by replacing photolithography steps with printing.

Finally, this invention relates to improvements in drivers forelectro-optic displays using an impulse sensitive electro-optic medium.

SUMMARY OF INVENTION

Accordingly, in one aspect this invention provides a process for formingat least one non-linear element on a flexible substrate. This processcomprises:

securing a flexible substrate on a substantially rigid carrier;

forming at least one non-linear element on the flexible substrate whileis flexible substrate is secured to the substantially rigid carrier; and

separating the flexible substrate and the at least one non-linearelement from the substantially rigid carrier.

This process of the invention may hereinafter for convenience be calledthe “rigid carrier” process.

In this rigid carrier process, the flexible substrate may comprise apolyimide layer. The flexible substrate may also comprise any one ormore of a moisture barrier layer, a reflective layer, a release layerand a dielectric capping layer. The reflective layer may comprises apolymeric material having reflective metal particles dispersed therein.

In one form of the rigid carrier process, the rigid carrier transmits atleast one wavelength of electromagnetic radiation, and the formation ofthe at least one non-linear element comprises at least one step in whichelectromagnetic radiation is transmitted through the substantially rigidcarrier. The rigid carrier may, for example be formed at least in partof glass.

The rigid carrier process is especially useful for forming backplanesfor electro-optic displays, i.e., in the rigid carrier process, the atleast one non-linear element may comprise at least one backplane for anelectro-optic display, and conveniently a plurality of discretebackplanes for electro-optic displays. Following formation of such aplurality of backplanes, both the flexible substrate and thesubstantially rigid carrier may be separated into a plurality ofseparate sections, each comprising one backplane, and thereafter in eachof the separate sections, the substantially rigid carrier separated fromthe flexible substrate and the backplane. After the formation of theplurality of separate sections, but before separation of thesubstantially rigid carrier from the flexible substrate in each section,each section may be subjected to attachment of an electrical connectorto the backplane and/or deposition of an electro-optic medium on thebackplane. Separation of the substantially rigid carrier from thebackplane is conveniently effected by radiation ablation of an ablatablelayer disposed between the substantially rigid carrier and thebackplane. A supporting layer may be attached to the flexible substrateafter separation of the flexible substrate from the substantially rigidcarrier.

Alternatively, after formation of a plurality of backplanes, theflexible substrate may be separated from the substantially rigidcarrier, and thereafter the flexible substrate separated into aplurality of separate sections, each comprising one backplane. In such aprocess, after separation of the substantially rigid carrier from theflexible substrate but before separation of the flexible substrate intothe sections, the flexible substrate may have an electrical connector toeach of the backplanes, and/or an electro-optic medium may be depositedon the backplanes. Again, separation of the substantially rigid carrierfrom the backplane is conveniently effected by radiation ablation of anablatable layer disposed between the substantially rigid carrier and thebackplane, and a supporting layer may be attached to the flexiblesubstrate after separation of the flexible substrate from thesubstantially rigid carrier.

In another variant of the rigid carrier process, following formation ofthe plurality of backplanes, a transfer substrate may be secured to theexposed surface of the flexible substrate carrying the backplanes, andthereafter the flexible substrate separated from the substantially rigidcarrier. Again separation of the substantially rigid carrier from theflexible substrate is conveniently effected by radiation ablation of anablatable layer disposed between the substantially rigid carrier and thebackplane, and the flexible substrate may be separated into a pluralityof sections each comprising one backplane, after separation of thesubstantially rigid carrier from the flexible substrate. The transfersubstrate may be removed from the flexible substrate prior to separationof the flexible substrate into the sections.

In all variants of the rigid transfer process, it is generally desirablethat the substantially rigid carrier have a coefficient of thermalexpansion which is at least as great as the coefficient of thermalexpansion of the flexible substrate.

The flexible substrate may be attached to the substantially rigidcarrier in various ways. For example, one of the substantially rigidcarrier and the flexible substrate may be provided with a plurality ofprojections and the other of the substantially rigid carrier and theflexible substrate provided with a plurality of aperture or recessesarranged to receive the projections, the flexible substrate beingsecured to the substantially rigid carrier by inserting the projectionsinto the apertures or recesses. Alternatively, the flexible substratemay be secured to the substantially rigid carrier by spot welding, ormagnetically.

In other aspect, this invention provides a method for forming atransistor array on a insulating substrate. This method comprises, inorder:

forming a plurality of gate electrodes on the substrate;

depositing over said gate electrodes a dielectric layer, a semiconductorlayer and a conductive layer;

patterning said conductive layer to form adjacent each gate electrode asource and drain electrode pair separated by a channel region, saidpatterning also forming a pixel electrode for each source and drainelectrode pair, said pixel electrode being electrically connected to oneof the source and drain electrodes;

covering said channel region of each transistor with an etch-resistantmaterial; and

etching the resultant structure using the etch-resistant material andthe exposed portions of the conductive layer as a mask, said etchingextending substantially through the semiconductor layer between adjacenttransistors.

This invention extends to a transistor array produced by this method,and to an electro-optic display comprising such a transistor array incombination with a plurality of pixel electrodes each of which isconnected to one of the source and drain electrodes of one transistor ofthe array, an electro-optic medium disposed adjacent the pixelelectrodes and at least one electrode on the opposed side of theelectro-optic medium from the pixel electrodes.

In another aspect, this invention provides a process for forming a diodeon a substrate, the process comprising:

depositing a first conductive layer on the substrate;

depositing a second patterned conductive layer over part of the firstconductive layer;

depositing a patterned dielectric layer over part of the firstconductive layer; and

etching the first conductive layer using the second patterned conductivelayer and the dielectric layer as an etch mask, thereby forming at leastone diode on the substrate.

Finally, this invention provides, in a process for driving animpulse-sensitive electro-optic display, which comprises applying toeach of the pixels of the display a voltage selected from within avoltage range for a time selected within a time range, the improvementwhich comprises providing an additional voltage spaced from said voltagerange, and applying the additional voltage to at least one pixel of thedisplay for a time within the time range.

BRIEF DESCRIPTION OF DRAWINGS

Preferred embodiments of the present invention will now be described,though by way of illustration only, with reference to the accompanyingdrawings, in which:

FIG. 1 is a graph showing the variation with wavelength of a film stackand resist useful in a rigid carrier process of the present invention;

FIGS. 2A–2E, 3A–3F and 4A–4F are schematic side elevations at variousstages of three preferred rigid carrier processes of the presentinvention;

FIGS. 5A–5D illustrate various types of rigid carriers which may be usedin rigid carrier processes of the present invention;

FIG. 6 illustrates a preferred method for bonding a substrate to acarrier in a rigid carrier process of the present invention;

FIG. 7 illustrates a second preferred method for bonding a substrate toa carrier in a rigid carrier process of the present invention;

FIG. 8 illustrates a peg and hole scheme which may be used for securinga flexible substrate to a substantially rigid carrier in a rigid carrierprocess of the present invention;

FIG. 9 illustrates an alternative form of peg and hole scheme which maybe used for securing a flexible substrate to a substantially rigidcarrier in a rigid carrier process of the present invention;

FIGS. 10A and 10B illustrate two different stages in a preferreddelamination step for use in a rigid carrier process of the presentinvention;

FIGS. 11 to 15 are schematic cross-sections through a substrate andtransistor at various stages during a process of the present inventionfor formation of the transistor;

FIGS. 16 and 17 are schematic cross-sections, similar to those of FIGS.11 to 15, illustrating two optional additional steps in the processillustrated in FIGS. 11 to 15;

FIGS. 18A to 18D are schematic side elevations showing various stages ofa process of the present invention for formation of a diode on asubstrate;

FIGS. 19 and 20 are schematic top plan views corresponding to the sideelevations of FIGS. 18B and 18D respectively; and

FIG. 21 is a schematic section through a second form of diode producedby a process of the present invention.

DETAILED DESCRIPTION

In the following detailed description, rigid carrier processes of theinvention will first be described with reference to FIGS. 1–10B. Next,processes for the formation of transistors by processes of the inventionwill be described with reference to FIGS. 11–17. Processes of theinvention for the formation of diodes and diode arrays will then bedescribed with reference to FIGS. 18A–21. Finally, methods of thepresent invention for driving electro-optic displays will be described.

Rigid Carrier Processes

As already mentioned this invention provides a rigid carrier process forforming a non-linear element, which may be a diode or a transistor, on aflexible substrate. The flexible substrate is secured to a substantiallyrigid carrier, at least one non-linear element is formed on the flexiblesubstrate, and then the flexible substrate and the at least onenon-linear element are separated from the substantially rigid carrier.

In the rigid carrier process of the present invention, the basic processflow is as follows. The first step is to provide the flexible substrateon the rigid carrier; this carrier is preferably formed of display gradeglass. The flexible substrate may be a pre-formed substrate, or may beformed in situ by coating a substrate-forming material on to thecarrier. For example, the flexible substrate may be formed by coating athick layer of polyimide on to a carrier. The flexible substrate mayconsist of only a single layer (typically a layer of polymer havingsufficient mechanical integrity) or may comprise multiple layers (films)that serve different functions. For example, a moisture barrier film maybe included in the substrate to prevent moisture absorption after thesubstrate and the non-linear elements have been separated from thecarrier. Alternatively or in addition, a reflective layer, such as asputtered or evaporated thin metallic film, may be included to block UVor other electromagnetic radiation.

As already indicated, in some cases the flexible substrate may include aablatable or other type of release layer. A simple polyimide or otherpolymeric layer may itself act as a release layer. However, in manycases it may be desirable to include a release layer separate from thepolymeric or other layer which provides the basic mechanical integrityof the flexible substrate. For example, a release layer may be formed ofamorphous silicon, in which case the amorphous silicon release layer ispreferably disposed adjacent the carrier when the flexible substrate issecured to the carrier. Alternatively, a polymeric film with UV (orother electromagnetic radiation) reflecting or absorbing particles maybe used as the release layer.

Thus, a wide range of structures are possible in the flexible substrateused in the present invention. Examples of such structures include(giving the layers in order, with the layer closest to the substantiallyrigid carrier first):

(a) a single polymeric layer, typically of polyimide;

(b) a polymeric release layer, typically of polyimide, a reflectivelayer (which may also serve as a moisture barrier) and a main polymericlayer (the reflective layer under the main polymeric layer preventsmoisture absorption after the carrier is separated from the flexiblesubstrate);

(c) an amorphous silicon release layer, a reflective layer (which mayalso serve as a moisture barrier) and a main polymeric layer; and

(d) a polymeric layer comprising a polymer, typically polyimide,containing reflective metal particles, and a main polymeric layer.

Any of these structures may have a dielectric capping layer on top ofthe main polymeric layer to prevent moisture and solvent absorptionduring the fabrication of the non-linear elements and/or to preventout-gassing during vacuum deposition steps. One suitable capping layercomprises 100–500 nm of silicon nitride deposited, for example by PECVD.

Polymers other than polyimide may of course be used in this process, butthe polymer should be able to withstand the temperatures used infabrication of non-linear elements (such as thin film transistors,“TFT's), which are typically about 150–350° C., and should also becompatible with the process employed to release the flexible substratefrom the substantially rigid carrier. One example of a suitablenon-polyimide material is benzocyclobutene, sold by Dow ChemicalCompany, Wilmington Del., under the Registered Trademark CYCLOTENE.

As already indicated many TFT and other non-linear element manufacturingprocesses use backside resist exposure (i.e., exposure of a photoresistlayer through the substrate) to form self-aligned structures. Thus, inmany rigid carrier processes of the present invention it may bedesirable to use a polymeric film that will allow a backside exposurestep to be used. The flexible substrate should be optimized to allowsradiation to pass therethrough the layers and expose photoresist coatedthereon. If, as is generally preferred, the flexible substrate is to bereleased from the rigid carrier by a radiation-induced release process,a backside exposure step requires that the layer which acts as therelease layer of the flexible substrate absorb radiation at thewavelength (λ_(r)) used in the release step, but that both this releaselayer and the other layers of the flexible substrate transmit radiationat the exposure wavelength (λ_(e)) used in the backside exposure step,as illustrated in FIG. 1 of the accompanying drawings. The releasewavelength λ_(r) will typically be in the range of 200–300 nm, forexample KrF radiation at about 256 nm or XeCl radiation at about 308 nm,while the exposure wavelength λ_(e) will typically be in the range of400–1200 nm, for example mercury H line radiation at about 405 nm or Gline radiation at about 436 nm. The wavelength, designated, λ_(r*), atwhich the absorption of the flexible substrate falls to zero must ofcourse be less than λ_(e).

In the rigid carrier process of the present invention, after theflexible substrate has been secured to the carrier, a TFT array, orother non-linear elements may be formed on the flexible substrate usingany standard process suitable for glass substrates. However, there aretwo principal variants of the rigid carrier process. In the firstvariant, illustrated in FIGS. 2A–2E, the flexible substrate(illustrated, for ease of illustration, as comprising only a singlepolymeric layer 202) is secured to the carrier 204 (FIG. 2A). TFT's,generally designated 206 (and shown as comprising only two layersalthough more layers may be present) and constituting backplanes for aplurality of separate electro-optic displays are formed on the secureflexible substrate 202, and the entire resultant structure is firstdiced to form individual backplanes, only one of which is shown in FIG.2B. A tab interconnect 208 is bonded to each backplane via a conductiveadhesive 210, and then a layer 212 of an electro-optic medium (shown inFIG. 2C as an encapsulated electrophoretic medium) is deposited on thetab-connected backplane. A front electrode 214, which will typically becarried on a second substrate (not shown) is provided on the opposedside of the layer 212 from the backplane to give the structure shown inFIG. 2C; at this point, the display may optionally be provided with acolor filter array (not shown), typically adjacent the front electrode214. In practice, the electro-optic medium is typically deposited on thefront electrode and its associated substrate, and the resultantelectro-optic medium/front electrode/substrate sub-assembly is laminatedto the backplane to form the structure of FIG. 2C.

At this point, the display is essentially complete, and the last mainstage in the process is separation of the display from the carrier 204.This is typically achieved by ablation at the interface between thecarrier 204 and the substrate 202 (as shown in FIG. 2D) using radiation216 which is directed through the carrier 204; the ablation ispreferably laser ablation using for example an excimer laser. Theablation releases the completed flexible display. Prior to release fromthe carrier 204, additional structures may be laminated to the displaymedium, such as a color filter array and/or a protective cover. Toprovide additional mechanical support to the released display, a supportlayer 218 (typically a low cost flexible polymeric film such as PES orpolycarbonate) may optionally be attached, preferably by lamination, tothe surface of the flexible substrate 202 from which the carrier 204 hasbeen separated. The support layer 208 can be a pre-formed layer, or maybe provided by applying a precursor material (for example, dip coatingan oligomer or pre-polymer solution) on to the relevant surface andcuring to form the support layer. The support layer 218 provides addedstrength and protection to the final display.

A major advantage of this variant of the process is that the rigidcarrier provides dimensional stability to the thin film array throughoutthe entire assembly process, thus allowing very fine alignment of acolor filter array, and very fine alignment of tab interconnects to thethin film structures.

The second variant, illustrated in FIGS. 3A–3F, allows the rigid carrierto be reused. The steps illustrated in FIGS. 3A and 3B are identical tothe corresponding steps shown in FIGS. 2A and 2B respectively. However,the remaining steps are different. Without prior dicing of the substrate202 and carrier 204, the electro-optic medium 312 (preferably anencapsulated electrophoretic medium which may include a color filterarray—not shown) and front electrode 314 are selectively bonded only tothe active regions of the TFT arrays (FIG. 3C), then the carrier 204 isseparated from the displays in the same way as previously described(FIG. 3D), and only then is the substrate 202 diced to provideindividual displays (FIG. 3E) and tab interconnects attached viaconductive adhesive 210 (FIG. 3F). Optionally, a support layer 218,formed in any of the ways previously described, may be attached afterremoval of the carrier 204, as illustrated in FIG. 3E.

In addition to allowing the rigid substrate to be reused, the process ofFIGS. 3A–3F allows the electro-optic medium can be simultaneouslydeposited on many displays at once. This deposition may be effected byusing a patterned electro-optic medium/front electrode/substratesub-assembly which has holes where the tab bonds are needed. Such aprocess provides an improvement over conventional liquid displaymanufacturing, which requires each backplane to be packagedindividually.

The present invention allows a flexible electro-optic display to bemanufactured in an existing glass fab with virtually no changes to thefacility. The fab could stay on-line and would be able to simultaneouslyproduce flexible and glass-based backplanes. This invention side stepsvirtually all of the most difficult and costly issues associated withdeveloping commercial flexible display manufacturing; i.e. fixturing,re-tooling, handling, contamination, substrate warp, surface roughnessissues, surface defect issues, and substrate dimensional stability.

FIGS. 4A–4F illustrate another variant of the rigid carrier process ofthe invention; in this variant, the flexible substrate is separated fromthe rigid carrier before the electro-optic medium is applied to theflexible substrate. The process of FIGS. 4A–4F begins in the same way asthe processes previously described, as illustrated in FIGS. 4A and 4B,which are identical to FIGS. 2A and 2B respectively.

However, the subsequent steps in the process of FIGS. 4A–4F aredifferent from those previously described. Following TFT fabrication, atransfer substrate 420 that has a tacky surface is adhered to the frontsurface (i.e., the surface remote from the carrier) of the TFT-carryingsubstrate (FIG. 4C). This transfer substrate 420 desirably has thefollowing properties: 1) sufficient flexibility to be rolled on andpeeled off repeatedly; 2) consistent and repeatable peel strength, highenough to support the flexible substrate for handling, but low enough soas to not damage the flexible substrate or TFT array upon peeling, 3)leaving no residue after peeling, 4) capable of being used numeroustimes, and 5) able to tolerate the lamination temperature required topermanently bond a flexible substrate to the backplane. An example of asuitable material is Gel-Film made by GelPak, LLC in Sunnyvale, Calif.The material is a specialized silicone based film laminated to aflexible PET backing material. After the transfer substrate 420 isattached, the substrate 202 is separated from the carrier 204 in themanner previously described (FIG. 4D). With the transfer substrate 420attached, the substrate 202 and attached TFT array can easily be handledand stored with minimal risk of damage. The next step (FIG. 4E) islaminating a flexible support 218, of any of the types previouslydescribed to the surface of the substrate 202 from which the carrier 204was removed. If the flexible substrate itself has sufficient strengthand mechanical integrity, this step may not be necessary. Next (FIG.4F), the substrate is diced to produce individual backplanes; just priorto this dicing, the transfer substrate 420 layer is peeled from thesubstrate 202. For a display application, the display medium 212, frontelectrode 214 and tab interconnect 208 are then attached.

It will be apparent to those skilled in the relevant arts that thepreferred rigid carrier processes described above make major demandsupon the techniques used for fixturing flexible substrates during TFTbackplane fabrication. Ideally, existing TFT manufacturing equipmentwill be able to handle the fixtured substrate without modifications.This would allow flexible substrates to be processed in existing TFTfabs with minimal retrofitting which will accelerate thecommercialization of flexible displays. There are described belowseveral different approaches: to bonding, welding and physicaltechniques, laser release, post-process thinning, and magneticfixturing.

For all fixturing schemes it is important that the fixtured substratematch traditional glass substrates in terms of weight, thickness, andmechanical properties such as sag. Deviation from “glass specifications”is likely to require significant modifications to existing TFTfabrication facilities.

In one fixturing approach, the substrate is bonded to a carrier usingeither an organic or inorganic bonding layer. The carrier may be a solidsheet of material as shown in FIG. 5A, a partial sheet of material asshown in FIG. 5B, or a frame as shown in FIGS. 5C and 5D. Useful carriermaterials include stainless steel, glass, metals, plastics, andceramics.

It is desirable for the coefficient of thermal expansion (CTE) of thecarrier to be equal to or slightly larger than the CTE of the substrate.If the CTE of the carrier and substrate are equal, there will not be anystress during thermal cycling and, in the case of a frame, the tensionof the substrate will be maintained during thermal cycling. If the CTEof the carrier is slightly larger than the CTE of the substrate, thesubstrate will be under tension during thermal cycling, which willprevent the substrate from sagging. Another requirement (if heat is usedto form the bond between the carrier and the substrate) is that the bondtemperature exceed the process temperature (T_(bond)>>T_(process)) sothat the bond remains intact during processing. Bonding can be performedby applying bond material to either the carrier or the substrate, or byusing sheet of bonding material (for example, a sheet of indium or apreformed epoxy sheet), and using heat and/or pressure to form the bond.FIG. 6 illustrates the bonding of a flexible substrate 602 to aframe-type rigid carrier 604 using a pre-patterned sheet 606 of bondingmaterial. Examples of materials useful in the bonding layer includealuminum, indium and other solders, and high temperature epoxy.

One preferred embodiment of the method shown in FIG. 6 is illustrated inFIG. 7. In this preferred embodiment, a stainless steel substrate 702 isused with a pre-patterned sheet 706 of aluminum as the bonding materialand a stainless steel frame 704 as the carrier, this frame is arranged,by engineering of its steel composition, to have a CTE slightly greaterthan that of the stainless steel substrate. Bonding is effected atapproximately 650° C. under pressure.

After TFT formation, the substrate may be released before display dicingor the displays may be diced without debonding the substrate, asdescribed above with reference to FIGS. 2 to 4. In both cases, it ispreferable to clean the frame and reuse it. Debonding may be performedby any convenient method, for example elevated temperature, chemicaletching, or ultraviolet exposure.

The substrate may also be secured to the rigid carrier by welding andother physical techniques. This approach is similar to the bondingapproach previously described except that the substrate is spot-weldedto the carrier or is physically held thereby. FIG. 8 shows one exampleof a carrier 804 that physically holds a substrate 802. The carrier 804is provided with four upstanding projections 806 and the substrate 802is provided with corresponding apertures 808 which can be receive theseprojections 806. This carrier 804 may also be used to tension thesubstrate 802 if the apertures 808 in the substrate 802 are arrangedslightly closer together than the projections 806 on the carrier 804.Any convenient form of fastener could also be used in the place of theprojections/apertures arrangement. Note that in the specific embodimentshown in FIG. 8, the projections 806 will extend above the plane of thesubstrate 802, and the resultant protrusion may cause problems duringprocessing. Liquid could also get trapped in the fixturing mechanism orbetween the substrate and carrier. The alternative embodiment shown inFIG. 9 (in which projections 906 extend diagonally upwardly from theedges of a domed carrier 904, and can be inserted into apertures (notshown) provided in the peripheral portions of a substrate 902, theseperipheral portions being bent downwardly over the edges of the domedcarrier 904) does not have any part of the fixture extending above theplane of the substrate 902, and may thus be preferred.

If the substrate is spot-welded to the carrier, the substrate could bediced to produce individual displays without separating it from thecarrier. If the substrate is clamped to the carrier or otherwisephysically secured, the substrate would normally be removed from thecarrier prior to the dicing step.

Various methods may be used to release the substrate from the carrier inthe rigid carrier process. Laser release has already been describedabove with reference to FIGS. 2 to 4. In the further embodiment shown inFIGS. 10A, and 10B, an adhesive layer 1006, typically of polyimide orother polymer, is used to secure a steel (or other metal) substrate 1002to a glass or other rigid carrier 1004. TFT's (not shown) are formed onthe exposed surface (the upper surface in FIG. 10A) of the steelsubstrate 1002 using the techniques described above and below, andthereafter the steel substrate 1002 is separated from the carrier 1004by application of radiation through the carrier, as illustrated in FIG.10B. This process maintains the mechanical advantages of a steelsubstrate (as opposed to attempting to handle an extremely fragilepolymer foil separate from a steel substrate).

Delamination can also be accomplished using a wet chemical process ifthe adhesive layer is soluble in the chemical. This technique desirablyrequires a method for delivering the chemical over the entire arearather than just at the edges. One approach is to machine holes orchannels in the carrier to assist with chemical delivery. Another optionis to use a porous substrate material or a porous bonding material toassist with chemical delivery.

Another technique for separating the substrate from the carrier is touse heat (a temperature close to the glass transition temperature of theadhesive layer) to weaken the adhesive-carrier interface. The substratecould then be peeled from the carrier. A precision tool (such as awedge) could also be used to separate the substrate from the carrier.

Since stainless steel substrates are magnetic or magnetizable, they canbe transported and fixtured during the TFT fabrication process by theuse of magnets.

The rigid carrier process of the invention may be carried out by formingthe TFT's on a thick, semi-rigid material, for example a polymer, andthen back-lapping or thinning the substrate after TFT formation. Suchthinning could be effected before or after separation of the individualdisplays from the process sheet.

Process for Forming Transistors with Reduced Number of Patterning Steps

For low-cost and simplified manufacturing, it is advantageous to reducethe number of process steps in the TFT fabrication process. This isespecially true for flexible TFT backplanes because of the substratehandling challenges and the potential for yield loss during handling andprocessing. For example, in a roll-to-roll manufacturing process forTFT's on flexible substrates, reducing the number of process steps willreduce the number of times that the roll has to be wound and unwound andthe number times that the substrate contacts the rollers. This willimprove yields.

This invention provides a process for forming a transistor array on aninsulating substrate; this process may use only three mask steps.

In the first step of a preferred form of this process, illustrated inFIG. 11, an insulating layer substrate is provided. A preferredsubstrate is preferred by depositing an insulating layer 1102, forexample of polyimide (see the aforementioned 2002/0019081) is depositedon to a steel or similar foil 1104 and a passivating layer 1106, forexample of silicon nitride, is deposited over the insulating layer 1104.A first metal layer is coated over the passivating layer 1106 andpatterned to form a gate electrode 1108. Next, as shown in FIG. 12,there are successively deposited a gate dielectric layer 1110, typicallyof silicon nitride, a semiconductor layer 1112 (preferably comprising alayer of amorphous silicon, α-Si, followed by a layer of n-dopedamorphous silicon, n+α-Si), and a second metal layer 1114. The resultantstructure is then etched, preferably by reactive ion etching (RIE)through the full thickness of the second metal layer 1114 and part waythrough the semiconductor layer 1112 (preferably using a time etch whichetches through the n+α-Si layer but not through the underlying α-Silayer; it is of course essential that this α-Si layer remain within thechannel region of the transistor), to produce the structure shown inFIG. 13, in which the large remaining portion of the second metal layerforms both the source 1116 and drain electrodes 1118 of the transistorand the pixel electrode 1120 which will, in the final form of thedisplay in which the transistor is intended to be used, lie adjacent anelectro-optic medium (not shown).

At this point, the semiconductor layer 1112 is continuous betweenadjacent transistors and thus, as indicated in FIG. 14, provides aleakage path 1122 between adjacent transistors. To remove this leakagepath 1122, a second passivating layer 1124, preferably of siliconnitride, is deposited over the whole surface of the substrate to producethe structure shown in FIG. 14, and thereafter a layer of photoresist(or other etch-resistant material, for example polyimide) is applied andpatterned so that the remaining portion 1126 (FIG. 15) of thephotoresist covers the portions of the second passivating layer 1124overlying the source 1116 and drain 1118 electrodes. Finally, theassembly is subjected to a second etching step, again preferably usingreactive ion etching, to produce the structure shown in FIG. 15; notethat this second etching step uses both the photoresist 1126 and thepixel electrode 1120 as a mask, and etches completely through thesemiconductor layer 1112, thus closing the leakage path 1122 throughthis layer and completely isolating adjacent transistors from eachother. Although the second etching step is illustrated in FIG. 15 asetching completely through the gate dielectric layer 1110, those skilledin the art of fabricating TFT's will appreciate that this is notstrictly necessary and that none, part or all of the thickness of thegate dielectric layer 1110 may be removed during this second etchingstep.

The photoresist 1126 or other etch-resistant material shown in FIG. 15may be removed, or allowed to remain to act as a light shield to reducelight-induced leakage through the channel region of the TFT.

FIGS. 16 and 17 illustrate two optional additional steps in the process.The photoresist may be reflowed by heating as shown in FIG. 16 so thatit flows into the gaps 1128 (FIG. 15) provided by the preceding etchingstep. Also, following such reflowing, a further layer of conductivematerial 1130 (FIG. 17), which may be a metal or a conductive polymermay be formed on top of the original pixel electrode 1120, and thisadditional conductive layer can extend over the transistor, thusproviding a “buried transistor” design.

In a variant of the process, contact pads may be formed on the substrateat the same time as, but spaced from, the gate electrodes 1108, thesecontact pads being positioned such that the final etching step exposesthe contact pads, thus rendering them available for connection to otherparts of the overall display. Also, in another variant of the process,the second passivating layer 1124 may be omitted.

This process of this invention achieves a patterned semiconductor layerwithout an additional mask step and thus, as compared with prior artprocesses which do not pattern the semiconductor layer, reduces thedegradation of performance caused by leakage through an unpatternedsemiconductor layer when the transistor array is in use.

Diodes with Printed Components

As already indicated, the cost of backplanes may be significantlyreduced by replacing photolithography with printing, and this aspect ofthe present invention relates to incorporating printing into themanufacture of diode matrix backplanes. Printing may be incorporatedinto electro-optic (especially encapsulated electrophoretic) displaysbecause the electro-optic medium does not require a planar surface, incontrast to liquid crystal displays, which require tight control of cellgaps on the order of 5–10 μm. Printed films are usually on the order of5–25 μm so printing is not suitable for liquid crystal displays. Theperformance of printed devices is typically worse than the performanceof devices fabricated using standard techniques (photolithography forexample). Compared to other display technologies, encapsulatedelectrophoretic and some other electro-optic media do not require highdrive current and may be able to tolerate the lower performance.

Metal-insulator metal (MIM) diodes are commonly used for active matrixbackplanes. The insulator is typically formed by anodizing a patternedmetal film. To make electrical contact to an anodized metal film, acontact hole is formed using photolithography and etching. To avoid thisphotolithography step, a conductive material such as carbon ink may bescreen-printed onto the metal prior to anodization. The printed materialshould be resistant to anodization. This invention provides a diodearray in which a conductive material is printed on a metal film toeliminate the need for a contact hole later in the process. Applicableprinting processes include screen, ink-jet, offset, intaglio, gravure,and flexographic (or any combination of these techniques). Applicablematerials include, but are not limited to, composite materials(screen-printing inks) and organic conductors.

The diode fabrication process may also be simplified by using a printeddielectric as an etch mask. The printed dielectric may remain on thedevice for isolation and capacitance reduction. This invention providesa process for forming a diode on a substrate; in this process, a printeddielectric is used as an etch mask. Applicable materials include bothorganic and inorganic dielectrics, as well as organic/inorganiccomposites.

The diode fabrication process may also be simplified by printing one ofthe electrodes of the diode or the pixel electrode. In some structures,the top electrode and pixel electrode may be printed at the same time.To achieve good diode performance, the electrode may consist of amultilayer stack. In this case, the conductive material would be printedonto a thin metal film which provides a high quality interface to thediode. The printed material would serve as a mask for etching the thinmetal film. This invention provides a diode structure in which anelectrode can be printed using a conductive material.

To improve the optical performance of a display, it may be desirable tobury the diode and select lines under a printed dielectric. The pixelelectrode (which may also be printed) would be tied to the diode througha via in the dielectric. This structure ensures that the electronic inkis only driven by the pixel electrode. Again, a printed dielectric isnot practical for liquid crystal displays because of the thickness ofprinted materials. Using standard processes (photolithography andetching) to achieve a buried structure adds cost and complexity whichdefeats the purpose of using diodes in the first place (low cost,simplified fabrication). This invention provides a diode structure inwhich the diode can be buried using a printed dielectric and printedpixel electrode.

FIGS. 18A–18D, 19 and 20 of the accompanying drawings illustrate apreferred process of the present invention for formation of diodes. Inthe first step shown in FIG. 18A, a metal layer 1802 is deposited overan insulating substrate, which may be of the polyimide-over-steel typedescribed above and comprise a polyimide layer 1804 formed on a steelsubstrate 1806. Next, as shown in side elevation in FIG. 18B and in topplan view in FIG. 19, a conductive material 1808 is printed in padregions and a dielectric material 1810 is printed in a line pattern.(the conductive material 1808 is omitted from FIGS. 18B–18D for ease ofillustration). (It will be appreciated that FIGS. 18A–18D onlyillustrate one half of the area shown in FIGS. 19 and 20, illustrating asingle area of dielectric material 1810 and its associated pixelelectrode 1814.) The next step of the process, shown in FIG. 18C, etchesthe metal layer 1802 using the dielectric 1810 and conductive materials1808 as a mask, and then anodizes the side walls of the resultant metalstrips 1812. Finally, as shown in side elevation in FIG. 18D and in topplan view in FIG. 20, a further (optional) metal layer is deposited, aconductive material is printed in the pattern required for pixelelectrodes 1814, and the metal layer (if present) is etched using theprinted conductive material as a mask.

FIG. 21 of the accompanying drawings illustrates one way in which aprinted dielectric 2102 and a printed pixel electrode 2104 may be usedwith a diode 2106 having a top electrode 2108 to provide a buried diodestructure.

Process for Driving Impulse-Sensitive Electro-Optic Display

As already indicated, and as discussed in more detail in theaforementioned copending application Ser. No. 10/065,795, manyelectro-optic displays (notably electrophoretic, rotating bichromalmember and electrochromic displays) respond not solely to appliedvoltage, but the product of voltage and time (current or impulse). Onemanner of achieving intermediate optical states is to modulate theimpulse seen by the display by using discrete voltage values, timevalues, or a combination of both. The data drivers (also called columndrivers) required to provide these many impulse levels can be expensive,especially if they require high voltages, and the cost increases withthe number of voltage/time steps required. In order to minimize displaycost for high voltage drivers with voltage modulation capability, it isadvantageous to concentrate the voltage levels in the region where theymost effectively allow the electro-optic effect of the display to becontrolled via impulse modulation.

One method of maximizing access to small voltage steps while maintaininghigh voltage resolution between steps is to provide a band of voltagesteps with an offset. For example, one could have 64 levels of voltageavailable, but instead of having those levels from 0–5V, one could havethem from 10–15V. The problem is that for a given number of voltagelevels and time steps, there are a finite mesh of voltage-time valuesthat can be achieved. If there are desired optical states that cannot beaddressed with these values, then those states cannot be displayed.

This invention provides additional voltage-time states that allow accessto “forbidden” states described above. The invention is described bymeans of the following example.

Assume that we have four voltage states (2 bit) and four time states,which allow 16 total voltage-time values shown in the table below.

time (s) voltage (v) 0.1 0.2 0.3 0.4 10 1.0 2.0 3.0 4.0 11 1.1 2.2 3.34.4 12 1.2 2.4 3.6 4.8 13 1.3 2.6 3.9 5.2

In the above example the range of v-t values is from 1 volt-second to5.2 volt seconds. If one requires a smaller (or larger) impulse, thevalues are not available. However if one adds in a discrete voltagevalue, one achieves four additional v-t states.

time (s) voltage (v) 0.1 0.2 0.3 0.4 3 0.3 0.6 0.9 1.2 10 1.0 2.0 3.04.0 11 1.1 2.2 3.3 4.4 12 1.2 2.4 3.6 4.8 13 1.3 2.6 3.9 5.2

In the second table, a 3V value has been added, which allows v-t statesfrom 0.3 to 5.2 volt-seconds. This can be repeated for additionaldiscrete values, above or below the original v-t range.

The reference voltages may be formed from a resistor network.

This invention allows one to eliminate blind spots in the v-t matrixwithout the addition of more bits of voltage modulation, which isexpensive. Instead, one creates a single additional state separated fromthe closely spaced voltage modulation group to fill in a series ofvalues unreachable with the voltage modulation group alone.

Electrophoretic and other electro-optic materials require precisecontrol of impulse values to achieve gray levels. Cost efficiencydictates that one cannot provide a continuous set of small steps from 0to maximum switching voltage, but only use values near the maximum.However, this does not allow one to achieve all of the voltage levelsneeded for the display of 4-bit grayscale. This simple modification tothe driver allows these states to be reached.

It will be apparent to those skilled in the art that numerous changescan be made in the specific embodiments of the present inventionsalready described without departing from the scope of the invention.Accordingly, the whole of the foregoing description is to be construedin an illustrative and not in a limitative sense.

1. A process for forming at least one non-linear element on a flexiblesubstrate, the process comprising: securing the flexible substrate on asubstantially rigid carrier, the substantially rigid carrier having acoefficient of thermal expansion which is at least as great as thecoefficient of thermal expansion of the flexible substrate; forming atleast one non-linear element on the flexible substrate while is flexiblesubstrate is secured to the substantially rigid carrier; and separatingthe flexible substrate and the at least one non-linear element from thesubstantially rigid carrier by radiation at the interface between theflexible substrate and the substantially rigid carrier.
 2. A processaccording to claim 1 wherein the flexible substrate comprises apolyimide layer.
 3. A process according to claim 1 wherein the flexiblesubstrate comprises any one or more of a moisture barrier layer, areflective layer, a release layer and a dielectric capping layer.
 4. Aprocess according to claim 3 wherein the reflective layer comprises apolymeric material having reflective metal particles dispersed therein.5. A process according to claim 1 wherein the substantially rigidcarrier transmits at least one wavelength of electromagnetic radiation,and the formation of the at least one non-linear element comprises atleast one step in which electromagnetic radiation is transmitted throughthe substantially rigid carrier.
 6. A process according to claim 1wherein the carrier is formed at least in part of glass.
 7. A processaccording to claim 1 wherein the at least one non-linear elementcomprises at least one backplane for an electro-optic display.
 8. Aprocess according to claim 7 wherein the at least one non-linear elementcomprises a plurality of discrete backplanes for electro-optic displays.9. A process according to claim 8 wherein, following formation of theplurality of backplanes, both the flexible substrate and thesubstantially rigid carrier are separated into a plurality of separatesections, each comprising one backplane, and thereafter in each of theseparate sections, the substantially rigid carrier is separated from theflexible substrate and the backplane.
 10. A process according to claim 9wherein, after the formation of the plurality of separate sections, butbefore separation of the substantially rigid carrier from the flexiblesubstrate in each section, each section is subjected to at least one ofattachment of an electrical connector to the backplane, and depositionof an electro-optic medium on the backplane.
 11. A process according toclaim 9 further comprising attaching a supporting layer to the flexiblesubstrate after separation of the flexible substrate from thesubstantially rigid carrier.
 12. A process according to claim 8 wherein,following formation of the plurality of backplanes, the flexiblesubstrate is separated from the substantially rigid carrier, andthereafter the flexible substrate is separated into a plurality ofseparate sections, each comprising one backplane.
 13. A processaccording to claim 12 wherein, after separation of the substantiallyrigid carrier from the flexible substrate but before separation of theflexible substrate into the sections, the flexible substrate issubjected to at least one of attachment of an electrical connector toeach of the backplanes, and deposition of an electro-optic medium on thebackplanes.
 14. A process for forming at least one non-linear element ona flexible substrate, the process comprising: securing the flexiblesubstrate on a substantially rigid cater; forming at least onenon-linear element on the flexible substrate while is flexible substrateis secured to the substantially rigid carrier, the non-linear elementcomprising a plurality of discrete backplanes for electro-opticdisplays; separating the flexible substrate and the substantially rigidcarrier into a plurality of separate sections, each comprising onebackplane; and thereafter, in each of the separate sections, separatingthe substantially rigid carrier from the flexible substrate by radiationablation at the interface between the substantially rigid carrier andthe flexible substrate.
 15. A process for forming at least onenon-linear element on a flexible substrate, the process comprising:securing the flexible substrate on a substantially rigid carrier;forming at least one non-linear element on the flexible substrate whileis flexible substrate is secured to the substantially rigid carrier; andseparating the flexible substrate and the at least one non-linearelement from the substantially rigid carrier by radiation ablation atthe interface between the substantially rigid carrier and the flexiblesubstrate.
 16. A process according to claim 15 wherein the flexiblesubstrate comprises a polyimide layer.
 17. A process according to claim15 wherein the flexible substrate comprises any one or more of amoisture barrier layer, a reflective layer, a release layer and adielectric capping layer.
 18. A process according to claim 17 whereinthe reflective layer comprises a polymeric material having reflectivemetal particles dispersed therein.
 19. A process according to claim 15wherein the substantially rigid carrier transmits at least onewavelength of electromagnetic radiation, and the formation of the atleast one non-linear element comprises at least one step in whichelectromagnetic radiation is transmitted through the substantially rigidcarrier.
 20. A process according to claim 15 wherein the carrier isformed at least in part of glass.
 21. A process according to claim 15wherein the at least one non-linear element comprises at least onebackplane for an electro-optic display.
 22. A process according to claim21 wherein the at least one non-linear element comprises a plurality ofdiscrete backplanes for electro-optic displays.
 23. A process accordingto claim 22 wherein, following formation of the plurality of backplanes,both the flexible substrate and the substantially rigid carrier areseparated into a plurality of separate sections, each comprising onebackplane, and thereafter in each of the separate sections, thesubstantially rigid carrier is separated from the flexible substrate andthe backplane.
 24. A process according to claim 23 wherein, after theformation of the plurality of separate sections, but before separationof the substantially rigid carrier from the flexible substrate in eachsection, each section is subjected to at least one of attachment of anelectrical connector to the backplane, and deposition of anelectro-optic medium on the backplane.